1. Field of the Invention
This invention relates, in general, to high pressure sodium lamps. More specifically, the invention relates to a high pressure sodium lamp which uses a translucent ceramic as a bulb with a pair of electrodes oppositely arranged, and a fill including a starting rare gas, mercury and sodium.
2. Description of the Prior Art
High pressure sodium lamps which use a translucent ceramic as a bulb are well known in the field of high efficiency discharge lamps. In such a lamp, the light permeable ceramic is made of a high density polycrystalline, such as alumina, or a metal oxide single crystal, such as ruby or sapphire.
In such a bulb using a translucent ceramic, since the ceramic has a high melting point, it is difficult to seal a pair of electrodes at the ends of the bulb in the normal manner by pinching the ends of the bulb under heat. In the case of aluminate ceramic, for example, sealing elements are used for sealing the opposite ends of the bulb through a glass solder. The sealing elements are made of the same material as the bulb, i.e., aluminate ceramic, or are made of a high melting point metal, such as, e.g., niobium and tantalum, whose thermal expansion coefficient is almost the same as the aluminate ceramic. A pair of electrodes are individually supported by each sealing element. More particularly, a metal tube made of, e.g., niobium, penetrates one of the sealing elements in an airtight relation, and one of the electrodes is fixed to the end of the metal tube which is positioned inside the bulb. During manufacture, the air in the bulb is exhausted through an exhausting hole in the metal tube, and the outer open end of the tube is sealed after the starting rare gas, excess amount of mercury and sodium are supplied to the bulb through the metal tube. Furthermore, the bulb comprising the construction described above is positioned in an outer envelope, the inside of which is held in a vacuous state.
In some of the lamps with the above-described construction, the lamp voltage thereof is sharply increased during lighting, and extinction of the arc between the electrodes occurs when the supply voltage is rapidly changed.
The inventors of the present invention discovered excess mercury and sodium condensed inside the metal tube in an inner area adjacent to the electrode and the exhaust hole. Since the outer end area ajdacent to the sealed outer end of the tube is the most cooled portion of the metal tube, such condensation normally should occur in that area. Therefore, the inventors concluded that the condensed sodium and mercury must have migrated from the outer area to the inner area.
Under the condition described above, since the temperature of the inner area of the metal tube is higher than that of the outer area of the metal tube, the mercury and sodium condensed in the inner area evaporate during lighting, and the vapor pressure of mercury and sodium in the bulb increases. In addition, sodium is easily reacted with the glass solder used as a sealing material, and alumina used as a bulb material, and thus it is consumed. As a result, the pressure of the mercury in the bulb partially increases, and thus the lamp voltage increases.
Next, the cause of the movement of mercury and sodium from the outer area of the tube toward the inner area of the tube will be discussed.
Materials which may be used for the metal tube of a sodium lamp are very limited. For example, the niobium commonly used for the tube is more difficult to process than iron, nickel or copper. As a result, a plurality of undesirable groove-shaped corrugations are produced, in the axis direction, on the inner surface of the metal tube during the manufacturing process. If excessive etching and annealing are used to remove these corrugations, the grain boundaries of the metal may appear on the inner surface of the tube after drawing, and as a result the inner surface of the tube may be too rough.
It is believed that a sodium-mercury amalgam condensed in the metal tube is melted into a fluid as the lamp is turned on. This fluid of sodium and mercury is believed to move toward the inner area of the tube (higher temperature portion) along the corrugations and the rough inner surface of the tube by capillary action.